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Preventing corrosion

Out on the water, your engine is constantly exposed to elements that can cause corrosion. Combating its damaging effects is critical.

Preventing corrosion

Galvanic corrosion

Galvanic corrosion occurs when two dissimilar metals, such as an aluminium gearcase and a stainless steel propeller, are immersed together in a conductive liquid. The resulting chemical reaction eats away at metal components, creating costly damage. Mercury Marine® designs a first line of defence into its products by using corrosion-resistant alloys, a multi-layer paint system and additional corrosion-resistant materials throughout the powertrain. By following a few easy additional steps, you can do even more to keep the effects of corrosion at bay, especially if you boat in saltwater.

A pro tip

Starting your engine when it is not in the water or connected to a water source, even briefly, can damage the seawater pump impeller. The soft impeller relies on water for lubrication. If the impeller is damaged the engine can overheat.

Flush, rinse and repeat

An outboard or sterndrive engine operated in saltwater should be flushed and rinsed after each use. This is easy to do and is an important way to inhibit corrosion. 

Flush it first

Flushing clears the cooling system of corrosive saltwater, and rinsing removes salt residue before corrosion can begin. Flushing after use in brackish, polluted or very silty freshwater is also recommended. MerCruiser® engines equipped with the optional SeaCore® freshwater cooling should be flushed to clear saltwater from the heat exchanger system and the exhaust.

There are several methods used to flush the cooling system. Check your owner’s manual for the flushing options for your engine.

Hose fitting

Many late-model Mercury® outboards have a dedicated flush fitting that can be used to connect a garden hose directly to the motor when the boat is on a trailer or in the water. Follow your operation and maintenance manual instructions to use the fitting.

Flush muff

A flush muff device can be used to flush an outboard that is not equipped with a dedicated hose fitting. A muff is also required for flushing a MerCruiser engine unless it has been fitted with an accessory engine flush kit (see a Mercury authorised dealer for more information). The muff fits around the gearcase and covers the cooling water inlets with rubber cups. A garden hose attached to the muff supplies freshwater. Follow your operation and maintenance manual instructions to use the flush muff.

Rinse away salt

Outboards

Rinse the outboard cowl, mounting bracket and trim ram area. Always lower the outboard to a vertical position before rinsing the cowl. Rinsing the cowl while the outboard is tilted can allow water to enter the air intake at the rear of the cowl. Next remove the cowl and the plastic flywheel cover and carefully rinse salt residue from the powerhead. Do not use high-pressure water or a spray nozzle, which can force water into electrical connections. Just let the water flow from the hose and take care to keep water away from the engine air intake and the alternator. Also rinse off the cowl latches, then allow the engine to dry.

Sterndrives

Rinse the outdrive unit with freshwater, including the trim ram area. Occasionally rinse salt residue from the engine using a hose and low water pressure. Remove the boat drain plug before rinsing to allow water to escape the bilge.

Mercury Corrosion Guard Engine Protect

Once the outboard powerhead or sterndrive engine is dry, apply a light coat of Mercury Corrosion Guard Engine Protect to sterndrive and inboard engine blocks and to under-cowl areas of outboard engines. Corrosion Guard forms a water-resistant barrier on painted and non‑painted metal surfaces. The clear film coating quickly dries to the touch and will not become sticky over long-term use. Take care to keep Corrosion Guard away from the air intake, alternator, and belts and pulleys. Spray Mercury Corrosion Guard Heavy Duty on the outdrive, gearcase, tilt and trim motor, exposed electrical connections and other surfaces that operate in the water. Avoid covering anodes. Use Mercury All-in-One Spotless Shine to remove unsightly water spots from fibreglass, metal, plastic and rubber surfaces, which helps keep your boat and engine shining like new.

Sacrificial anodes

Anodes, often called “zincs” and usually located on the gearcase, transom bracket or trim rams of a Mercury outboard or MerCruiser sterndrive, are designed to protect aluminium components from galvanic corrosion, especially when using a stainless steel propeller. A sacrificial anode is made of a more-vulnerable material and is intended to corrode more easily, or “sacrifice” itself, to protect the adjacent components. A common misconception among some boat owners is that non-corroded anodes are a good sign that corrosion is under control. In fact, the opposite is true. If your anodes show no signs of wear or corrosion, that may be an indication that damaging corrosion is occurring on other more critical parts.

Check your operation and maintenance manual for the location of anodes on your Mercury outboard or MerCruiser sterndrive.

Mercury recommends replacement when about half of the anode has been lost to corrosion.

It is good practice to frequently inspect sacrificial anodes, especially if you operate your boat in saltwater.

To get the best protection, avoid inexpensive aftermarket anodes. You can count on a genuine Mercury replacement anode to be made with the highest-quality, most-effective alloy.

Anodes look like a small grey block and are usually secured with a single bolt.

Anodes must be in direct contact with the water to work properly, so never paint or apply other coatings over them.

Anodes may become coated with a thin film of scum in some water conditions. This film can be removed with a stiff brush.